Thermal mass flow sensors operate by maintaining a temperature difference between two elements mounted in a mass flow passageway. A heating element is generally provided to heat one of the elements, and the temperatures of the elements are monitored. A mass flow, such as a mass of air, moving through the passageway and over the two elements cools the heated element. Large mass flows cool the heated element to a greater extent than do small mass flows. The amount of power required to maintain a given temperature difference therefore provides an indication of the mass flow.
Known thermal mass flow sensors suffer from several shortcomings. For example, with many designs, there is a non-linear relationship between the flow rate and the power required to maintain a temperature difference. Moreover, some sensors require significant power to operate, and many have slow response times. And, while a particular sensor may function adequately in a given environment, it is difficult to replicate the characteristics of that sensor and to make multiple sensors having the same output characteristics. Thus, recalibration is required each time a sensor is replaced. It is therefore desirable to provide a thermal mass flow sensor that addresses these and other shortcomings.